India’s GPS version will be a boon for other nations as well

On Thursday, the Indian Space Research Organisation (Isro) launched the sixth navigation satellite of the Indian Regional Navigation Satellite System (IRNSS). The system consists of a constellation of seven satellites and, once operational, it will provide us our own version of a Global Positioning System (GPS). It can also be used by people living in countries that are within 1,500 km of India. IRNSS will provide two types of services – a Standard Positioning Service (SPS) which will be available for everyone, and Restricted Service (RS), an encrypted service provided only to authorised users.

What is GPS?

GPS is a space-based radio navigation system first developed by the US Department of Defence for military use and now available to civilian users worldwide. For anyone with a GPS receiver, the system provides location and time. Apart from the GPS, the Russians maintain a similar GLONASS (Global Navigation Satellite System). The European Union is launching its own version of GPS, known as Galileo, and the Chinese are expanding their local navigation system BeiDou to a global level.

What are the components of these positioning systems?

The GPS is based on a constellation of 24 medium Earth orbit satellites that transmit microwave signals to the GPS receiver. From these signals, the receiver calculates the latitude, longitude, altitude and time for each specific user. The Russian system has 29 satellites in orbit. The proposed EU and Chinese systems will be based on 30 and 35 satellites, respectively.

What is the basic principle of such positioning systems?

These systems work on triangulation to find a location. Suppose you are somewhere in India but don’t know exactly where. A passer-by tells you that you are 115 km from Chittorgarh. You now know you are somewhere on a circle around Chittorgarh with a radius of 115 km. Another person tells you that you are 185 km from Mount Abu. Now you have two circles and, mathematically, two circles have two intersection points that could be near Udaipur or Rajsamand. Now a third person tells you that you are 262 km from Ahmedabad. Mathematically, three circles can intersect at only one point and hence you can deduce that you are in Udaipur. GPS uses a similar principle to locate positions.

How does it work?

The original GPS design consisted of 27 satellites, of which 24 were operational and three were for backup purposes. Each of these was making two complete rotations of the Earth every day. The satellites were so arranged that from any place on Earth at least four were visible. Three satellites would have been enough, if only three variables – latitude, longitude and altitude – were involved. The fourth variable was introduced since the satellites are moving at a high speed and minor time lags between the received signals of two satellites can cause a large positional error.